Process module for increasing the response of backside illuminated photosensitive imagers and associated methods

ABSTRACT

Backside illuminated photosensitive devices and associated methods are provided. In one aspect, for example, a backside-illuminated photosensitive imager device can include a semiconductor substrate having multiple doped regions forming a least one junction, a textured region coupled to the semiconductor substrate and positioned to interact with electromagnetic radiation where the textured region includes surface features sized and positioned to facilitate tuning to a preselected wavelength of light, and a dielectric region positioned between the textured region and the at least one junction. The dielectric region is positioned to isolate the at least one junction from the textured region, and the semiconductor substrate and the textured region are positioned such that incoming electromagnetic radiation passes through the semiconductor substrate before contacting the textured region. Additionally, the device includes an electrical transfer element coupled to the semiconductor substrate to transfer an electrical signal from the at least one junction.

PRIORITY DATA

This application claims the benefit of U.S. Provisional Application Ser.No. 61/495,243, filed on Jun. 9, 2011, which is incorporated herein byreference.

BACKGROUND

The interaction of light with semiconductor materials has been asignificant innovation. Silicon imaging devices are used in varioustechnologies, such as digital cameras, optical mice, video cameras, cellphones, and the like. Charge-coupled devices (CCDs) were widely used indigital imaging, and were later improved upon by complementarymetal-oxide-semiconductor (CMOS) imagers having improved performance.Many traditional CMOS imagers utilize front side illumination (FSI). Insuch cases, electromagnetic radiation is incident upon the semiconductorsurface containing the CMOS devices and circuits. Backside illuminationCMOS imagers have also been used, and in many designs electromagneticradiation is incident on the semiconductor surface opposite the CMOSdevices and circuits. CMOS sensors are typically manufactured fromsilicon and can covert visible incident light into a photocurrent andultimately into a digital image. Silicon-based technologies fordetecting infrared incident electromagnetic radiation have beenproblematic, however, because silicon is an indirect bandgapsemiconductor having a bandgap of about 1.1 eV. Thus the absorption ofelectromagnetic radiation having wavelengths of greater than about 1100nm is, therefore, very low in silicon.

SUMMARY

The present disclosure provides backside-illuminated photosensitiveimager devices and associated methods. In one aspect, for example, abackside-illuminated photosensitive imager device can include asemiconductor substrate having multiple doped regions forming a leastone junction, a textured region coupled to the semiconductor substrateand positioned to interact with electromagnetic radiation, wherein thetextured region includes surface features sized and positioned tofacilitate tuning to a preselected wavelength of light, and a dielectricregion positioned between the textured region and the at least onejunction. The dielectric region is positioned to isolate the at leastone junction from the textured region, and the semiconductor substrateand the textured region are positioned such that incomingelectromagnetic radiation passes through the semiconductor substratebefore contacting the textured region. Additionally, the device includesan electrical transfer element coupled to the semiconductor substrate totransfer an electrical signal from the at least one junction. In someaspects, the dielectric region is positioned to physically isolate thetextured region from the at least one junction. In other aspects, thedielectric region is positioned to electrically isolate the texturedregion from the at least one junction.

In one aspect, the surface features have an average center-to-centerdistance of one half wavelength of the preselected wavelength of light,multiples of one half wavelength of the preselected wavelength of light,or at least one half wavelength of the preselected wavelength of light,wherein the preselected wavelength in this context is scaled by therefractive index of the surrounding material. In another aspect, thecenter-to-center distance of the features is substantially uniformacross the textured region. In a further aspect, the surface featureshave an average height of about a multiple of a quarter wavelength ofthe preselected wavelength of light, wherein the preselected wavelengthis scaled by the refractive index of the surrounding material. Inanother aspect, the surface features can be sized and positioned toreduce specular reflection.

Additional regions and/or structures can be included in various devicesaccording to aspects of the present disclosure. In some aspects, forexample, the device can include a reflecting region coupled to thetextured region opposite the dielectric region and positioned to reflectelectromagnetic radiation passing through the textured region backthrough the textured region. Various reflective materials can beincluded in the reflecting region including, without limitation, a Braggreflector, a metal reflector, a metal reflector over a dielectricmaterial, and the like, including combinations thereof. In some aspects,one or more dielectric layers are positioned between the reflectingregion and the textured region. In another aspect, a lens can beoptically coupled to the semiconductor substrate and positioned to focusincident electromagnetic radiation into the semiconductor substrate.

The preselected wavelength of light can be any wavelength or wavelengthdistribution. In one aspect, for example, the preselected wavelength oflight can be in the near infrared or infrared range. In another aspect,the preselected wavelength of light can be greater than or equal toabout 800 nm.

In another aspect, one or more anti-reflective layers can be depositedon the semiconductor substrate at a surface opposite the at least onejunction such that incident light passes through the anti-reflectivelayer prior to contacting the semiconductor substrate. Additionally, ina further aspect at least one isolation feature can be formed in thesemiconductor substrate, where the at least one isolation feature ispositioned to reflect light impinging thereon back into thesemiconductor substrate.

The present disclosure additionally provides various methods of making abackside-illuminated photosensitive imager device. One such method caninclude forming at least one junction at a surface of a semiconductorsubstrate, forming a dielectric region over the at least one junction,and forming a textured region over the dielectric region. The texturedregion can include surface features sized and positioned to facilitatetuning to a preselected wavelength of light. The dielectric regionisolates the at least one junction from the textured region, and thesemiconductor substrate and the textured region are positioned such thatincoming electromagnetic radiation passes through the semiconductorsubstrate before contacting the textured region. The method can alsoinclude coupling an electrical transfer element to the semiconductorsubstrate such that the electrical transfer element is operable totransfer an electrical signal from the at least one junction.

Various techniques are contemplated for forming the textured region, andany technique useful for such a process is considered to be within thepresent scope. Non-limiting examples of such techniques include plasmaetching, reactive ion etching, porous silicon etching, lasing, chemicaletching, nanoimprinting, material deposition, selective epitaxialgrowth, lithography, and the like, including combinations thereof. Inone specific aspect, the forming of the textured region can includedepositing a mask on the dielectric region, etching the dielectricregion through the mask to form surface features, and removing the maskfrom the dielectric region. The surface features can be further etchedto round exposed edges. In yet another specific aspect, forming thetextured region further includes depositing a first semiconductormaterial on the dielectric region, texturing the first semiconductormaterial to form a mask, depositing a second semiconductor material onthe mask, and etching the second semiconductor material to form thetextured region. In one specific aspect, texturing the secondsemiconductor material further includes etching the second semiconductormaterial to form a plurality of scallops pointing toward thesemiconductor substrate. A variety of first and second semiconductormaterials are contemplated, and any suitable material is considered tobe within the present scope. In one aspect, however, the first andsecond semiconductor materials include silicon, polysilicon, amorphoussilicon, or the like, including combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantage of the presentinvention, reference is being made to the following detailed descriptionof preferred embodiments and in connection with the accompanyingdrawings, in which:

FIG. 1 is a schematic diagram of a four transistor active pixel sensor(APS) of a CMOS imager in accordance with one aspect of the presentdisclosure;

FIG. 2 is a schematic view of a photosensitive device in accordance withanother aspect of the present disclosure;

FIG. 3 is a schematic view of a pixel of a photosensitive device with aBSI photodiode in accordance with yet another aspect of the presentdisclosure;

FIG. 4 illustrates light trapping in a thin semiconductor layer with aplanar illuminated side and a textured opposing side in accordance witha further aspect of the present disclosure;

FIG. 5 is a graph showing calculated absorptance of infrared radiationin a thin silicon photodetector with light trapping and differentamounts of light reflected back from the illuminated surface;

FIG. 6 is a schematic view of a photosensitive device in accordance withanother aspect of the present disclosure;

FIG. 7 is a depiction of a method of making a photosensitive imagerdevice in accordance with another aspect of the present disclosure;

FIG. 8a is a schematic view of a photosensitive device in accordancewith yet another aspect of the present disclosure;

FIG. 8b is a schematic view of a photosensitive device in accordancewith yet another aspect of the present disclosure;

FIG. 9a is a top view of a textured region in accordance with one aspectof the present disclosure;

FIG. 9b is a cross-sectional view of a textured surface in accordancewith an aspect of the present disclosure;

FIG. 10a is a cross-sectional view of a photosensitive device duringmanufacture in accordance with another aspect of the present disclosure;

FIG. 10b is a cross-sectional view of a photosensitive device duringmanufacture in accordance with another aspect of the present disclosure;

FIG. 10c is a cross-sectional view of a photosensitive device duringmanufacture in accordance with another aspect of the present disclosure;

FIG. 11a is a cross-sectional view of a photosensitive device duringmanufacture in accordance with yet another aspect of the presentdisclosure;

FIG. 11b is a cross-sectional view of a photosensitive device duringmanufacture in accordance with yet another aspect of the presentdisclosure;

FIG. 11c is a cross-sectional view of a photosensitive device duringmanufacture in accordance with yet another aspect of the presentdisclosure;

FIG. 11d is a cross-sectional view of a photosensitive device duringmanufacture in accordance with yet another aspect of the presentdisclosure; and

FIG. 12 is a depiction of a method of making a photosensitive imagerdevice in accordance with another aspect of the present disclosure.

DETAILED DESCRIPTION

Before the present disclosure is described herein, it is to beunderstood that this disclosure is not limited to the particularstructures, process steps, or materials disclosed herein, but isextended to equivalents thereof as would be recognized by thoseordinarily skilled in the relevant arts. It should also be understoodthat terminology employed herein is used for the purpose of describingparticular embodiments only and is not intended to be limiting.

Definitions

The following terminology will be used in accordance with thedefinitions set forth below.

It should be noted that, as used in this specification and the appendedclaims, the singular forms “a,” and, “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a dopant” includes one or more of such dopants andreference to “the layer” includes reference to one or more of suchlayers.

As used herein, “tuning” refers to selectively enhancing a device for aproperty of light at a desired wavelength or range of wavelengths. Inone aspect, a property of light can be absorptance, quantum efficiency,polarization, and the like.

As used herein, the term “textured surface” refers to a surface having atopology with nano- to micron-sized surface variations formed by theirradiation of laser pulses or other texturing methods. One non-limitingexample of other texturing methods can include chemical etching. Whilethe characteristics of such a surface can be variable depending on thematerials and techniques employed, in one aspect such a surface can beseveral hundred nanometers thick and made up of nanocrystallites (e.g.from about 10 to about 50 nanometers) and nanopores. In another aspect,such a surface can include micron-sized structures (e.g. about 500 nm toabout 60 μm). In yet another aspect, the surface can include nano-sizedand/or micron-sized structures from about 5 nm and about 500 μm.

As used herein, the terms “surface modifying” and “surface modification”refer to the altering of a surface of a semiconductor material using avariety of surface modification techniques. Non-limiting examples ofsuch techniques include plasma etching, reactive ion etching, poroussilicon etching, lasing, chemical etching (e.g. anisotropic etching,isotropic etching), nanoimprinting, material deposition, selectiveepitaxial growth, and the like, including combinations thereof. In onespecific aspect, surface modification can include processes usingprimarily laser radiation or laser radiation in combination with adopant, whereby the laser radiation facilitates the incorporation of thedopant into a surface of the semiconductor material. Accordingly, in oneaspect surface modification includes doping of a substrate such as asemiconductor material.

As used herein, the term “target region” refers to an area of asubstrate that is intended to be doped or surface modified. The targetregion of the substrate can vary as the surface modifying processprogresses. For example, after a first target region is doped or surfacemodified, a second target region may be selected on the same substrate.

As used herein, the term “fluence” refers to the amount of energy from asingle pulse of laser radiation that passes through a unit area. Inother words, “fluence” can be described as the energy surface density ofone laser pulse.

As used herein, the term “detection” refers to the sensing, absorption,and/or collection of electromagnetic radiation.

As used herein, the term “backside illumination” refers to a devicearchitecture design whereby electromagnetic radiation is incident on asurface of a semiconductor material that is opposite a surfacecontaining the device circuitry. In other words, electromagneticradiation is incident upon and passes through a semiconductor materialprior to contacting the device circuitry.

As used herein, the term “substantially” refers to the complete ornearly complete extent or degree of an action, characteristic, property,state, structure, item, or result. For example, an object that is“substantially” enclosed would mean that the object is either completelyenclosed or nearly completely enclosed. The exact allowable degree ofdeviation from absolute completeness may in some cases depend on thespecific context. However, generally speaking the nearness of completionwill be so as to have the same overall result as if absolute and totalcompletion were obtained. The use of “substantially” is equallyapplicable when used in a negative connotation to refer to the completeor near complete lack of an action, characteristic, property, state,structure, item, or result. For example, a composition that is“substantially free of” particles would either completely lackparticles, or so nearly completely lack particles that the effect wouldbe the same as if it completely lacked particles. In other words, acomposition that is “substantially free of” an ingredient or element maystill actually contain such item as long as there is no measurableeffect thereof.

As used herein, the term “about” is used to provide flexibility to anumerical range endpoint by providing that a given value may be “alittle above” or “a little below” the endpoint.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary.

Concentrations, amounts, and other numerical data may be expressed orpresented herein in a range format. It is to be understood that such arange format is used merely for convenience and brevity and thus shouldbe interpreted flexibly to include not only the numerical valuesexplicitly recited as the limits of the range, but also to include allthe individual numerical values or sub-ranges encompassed within thatrange as if each numerical value and sub-range is explicitly recited. Asan illustration, a numerical range of “about 1 to about 5” should beinterpreted to include not only the explicitly recited values of about 1to about 5, but also include individual values and sub-ranges within theindicated range. Thus, included in this numerical range are individualvalues such as 2, 3, and 4 and sub-ranges such as from 1-3, from 2-4,and from 3-5, etc., as well as 1, 2, 3, 4, and 5, individually.

This same principle applies to ranges reciting only one numerical valueas a minimum or a maximum. Furthermore, such an interpretation shouldapply regardless of the breadth of the range or the characteristicsbeing described.

The Disclosure

Electromagnetic radiation can be present across a broad wavelengthrange, including visible range wavelengths (approximately 350 nm to 800nm) and non-visible wavelengths (longer than about 800 nm or shorterthan 350 nm). The infrared spectrum is often described as including anear infrared portion of the spectrum including wavelengths ofapproximately 800 nm to 1300 nm, a short wave infrared portion of thespectrum including wavelengths of approximately 1300 nm to 3micrometers, and a mid to long wave infrared (or thermal infrared)portion of the spectrum including wavelengths greater than about 3micrometers up to about 30 micrometers. These are generally andcollectively referred to herein as “infrared” portions of theelectromagnetic spectrum unless otherwise noted.

Traditional silicon photodetecting imagers have limited lightabsorption/detection properties. For example, such silicon baseddetectors are mostly transparent to infrared light. While other mostlyopaque materials (e.g. InGaAs) can be used to detect infraredelectromagnetic radiation having wavelengths greater than about 1000 nm,silicon is still commonly used because it is relatively cheap tomanufacture and can be used to detect wavelengths in the visiblespectrum (i.e. visible light, 350 nm-800 nm). Traditional siliconmaterials require substantial path lengths to detect photons havingwavelengths longer than approximately 700 nm. While visible light can beabsorbed at relatively shallow depths in silicon, absorptance of longerwavelengths (e.g. 900 nm) in silicon of a standard wafer depth (e.g.approximately 750 μm) is poor if at all.

The devices of the present disclosure increase the absorptance ofsemiconductor materials by increasing the propagation path length forlonger wavelengths as compared to traditional materials. The absorptiondepth in conventional silicon detectors is the depth into silicon atwhich the radiation intensity is reduced to about 36% of the value atthe surface of the semiconductor. The increased propagation path lengthresults in an apparent reduction in the absorption depth, or a reducedapparent or effective absorption depth. For example, the effectiveabsorption depth of silicon can be reduced such that these longerwavelengths can be absorbed in material thicknesses of less than orequal to about 850 μm. In other words, by increasing the propagationpath length, these devices are able to absorb longer wavelengths(e.g. >1000 nm for silicon) within a thinner semiconductor material. Inaddition to decreasing the effective absorption depth, the response rateor response speed can also be increased by using thinner semiconductormaterials.

Accordingly, backside-illuminated (BSI) photosensitive imager devicesand associated methods are provided. Such devices provide, among otherthings, enhanced response in the infrared light portion of theelectromagnetic spectrum and improved response and quantum efficiency inconverting electromagnetic radiation to electrical signals. Quantumefficiency can be defined as the percentage of photons that areconverted into electrons and collected by a sensing circuit. There aretwo types of QE, Internal QE (IQE) and External QE (EQE). The EQE isalways lower than the IQE since there will inevitably be recombinationeffects and optical losses (e.g. transmission and reflection losses).One reason for improved performance with BSI is a higher fill factor or,in other words, the percentage if incident light that is incident on aphotosensitive region of the device. The various metal layers on top ofa front side-illuminated sensor (FSI) limit the amount of light that canbe collected in a pixel. As pixel sizes get smaller, the fill factorgets worse. BSI provides a more direct path for light to travel into thepixel, thus avoiding light blockage by the metal interconnect anddielectric layers on the top-side of the semiconductor substrate.

The present disclosure additionally provides BSI broadbandphotosensitive diodes, pixels, and imagers capable of detecting visibleas well as infrared electromagnetic radiation, including associatedmethods of making such devices. A photosensitive diode can include asemiconductor substrate having multiple doped regions forming at leastone junction, a textured region coupled to the semiconductor substrateand positioned to interact with electromagnetic radiation where thetextured region includes surface features sized and positioned tofacilitate tuning to a preselected wavelength of light, and a dielectricregion positioned between the textured region and the at least onejunction. The dielectric region is positioned to isolate the at leastone junction from the textured region, and the semiconductor substrateand the textured region are positioned such that incomingelectromagnetic radiation passes through the semiconductor substratebefore contacting the textured region. It should be noted that, in someaspects, the dielectric region can be a passivation region.

In one aspect the multiple doped regions can include at least onecathode region and at least one anode region. In some aspects, dopedregions can include an n-type dopant and/or a p-type dopant as isdiscussed below, thereby creating a p-n junction. In other aspects, aphotosensitive device can include an i-type region to form a p-i-njunction.

A photosensitive pixel can include a semiconductor substrate havingmultiple doped regions forming at least one junction, a textured regioncoupled to the semiconductor substrate and positioned to interact withelectromagnetic radiation where the textured region includes surfacefeatures sized and positioned to facilitate tuning to a preselectedwavelength of light, and a dielectric region positioned between thetextured region and the at least one junction. The dielectric region ispositioned to isolate the at least one junction from the texturedregion, and the semiconductor substrate and the textured region arepositioned such that incoming electromagnetic radiation passes throughthe semiconductor substrate before contacting the textured region.Additionally, the photosensitive pixel also includes an electricaltransfer element coupled to the semiconductor substrate and operable totransfer an electrical signal from the at least one junction. Aphotosensitive imager can include multiple photosensitive pixels.Additionally, an electrical transfer element can include a variety ofdevices, including without limitation, transistors, sensing nodes,transfer gates, transfer electrodes, and the like.

Photosensitive or photo detecting imagers include photodiodes or pixelsthat are capable of absorbing electromagnetic radiation within a givenwavelength range. Such imagers can be passive pixel sensors (PPS),active pixel sensors (APS), digital pixel sensor imagers (DPS), or thelike, with one difference being the image sensor read out architecture.For example, a semiconducting photosensitive imager can be a three orfour transistor active pixel sensor (3T APS or 4T APS). Variousadditional components are also contemplated, and would necessarily varydepending on the particular configuration and intended results. As anexample, a 4T configuration as is shown in FIG. 1 can additionallyinclude, among other things, a transfer gate 102, a reset transistor104, a source follower 106, a row select transistor 108, and aphotodiode sensor 110. Additionally, devices having greater than 4transistors are also within the present scope. In one aspect,photosensor diode 110 can be a conventional pinned photodiode as used incurrent state of the art complimentary metal-oxide-semiconductor (CMOS)imagers.

Photosensitive imagers can be front side illumination (FSI) or back sideillumination (BSI) devices. In a typical FSI imager, incident lightenters the semiconductor device by first passing by transistors andmetal circuitry. The light, however, can scatter off of the transistorsand circuitry prior to entering the light sensing portion of the imager,thus causing optical loss and noise. A lens can be disposed on thetopside of a FSI pixel to direct and focus the incident light to thelight sensing active region of the device, thus partially avoiding thecircuitry. In one aspect the lens can be a micro-lens. In one aspect,for example, incident light enters the device via the light sensingportion and is mostly absorbed prior to reaching the circuitry. BSIdesigns allow for smaller pixel architecture and a higher fill factorfor the imager. It should also be understood that devices according toaspects of the present disclosure can be incorporated into CMOS imagerarchitectures, charge-coupled device (CCD) imager architectures, and thelike.

In one aspect, as is shown in FIG. 2, a BSI photosensitive diode 200 caninclude a semiconductor substrate 202 having multiple doped regions 204,206 forming at least one junction, and a textured region 208 coupled tothe semiconductor substrate 202 and positioned to interact withelectromagnetic radiation 212. The multiple doped regions 204, 206 canhave the same doping profile or different doping profiles, depending onthe device. While the device shown in FIG. 2 contains three dopedregions, it should be noted that other aspects containing one or moredoped regions are considered to be within the present scope.Additionally, the semiconductor substrate 202 can be doped, and thus canbe considered to be a doped region in some aspects. A dielectric region210 is positioned between the textured region 208 and the at least onejunction. In one aspect, the dielectric region can have a thickness inthe range of about 1 nm to about 2 μm. In another aspect, the dielectricregion can have a thickness in the range of about 10 nm to about 100 nm.In yet another aspect the dielectric region can have a thickness of lessthan about 50 nm. The dielectric region 210 can be positioned to isolatethe at least one junction from the textured region 208, and thesemiconductor substrate 202 and the textured region 208 can bepositioned such that incoming electromagnetic radiation 212 passesthrough the semiconductor substrate 202 before contacting the texturedregion 208. The photosensitive diode is backside illuminated byelectromagnetic radiation 212 that is incident on the side of thesemiconductor substrate 202 opposite the multiple doped regions 204,206.

In another aspect, as is shown in FIG. 3, a BSI photosensitive imagerdevice 300 is provided. The BSI photosensitive imager device can includea semiconductor substrate 302 having multiple doped regions 304, 306forming a least one junction, and a textured region 308 coupled to thesemiconductor substrate 302 and positioned to interact withelectromagnetic radiation 310. A dielectric region 312 can be positionedbetween the textured region 308 and the at least one junction to isolatethe at least one junction from the textured region. The semiconductorsubstrate 302 and the textured region 308 are positioned such thatincoming electromagnetic radiation 310 passes through the semiconductorsubstrate 302 before contacting the textured region 308. Additionally,in some aspects an optional dielectric region or reflecting region 314can be coupled to the textured region 308. An electrical transferelement 316 can be coupled to the semiconductor substrate 302 totransfer an electrical signal from the at least one junction. Side wallinsulators 318 and 320 can also be formed about the transfer element 316and also around the dielectric and textured regions 312, 314,respectively, to facilitate proper spacing away from the transferelement 316. Additionally, a drain junction region 322 can beelectrically coupled to the transfer element 316 to receive chargetransferred thereto by the transfer element.

The various devices according to aspects of the present disclosure canexhibit increased quantum efficiency over traditional photosensitivedevices. Any increase in the quantum efficiency makes a large differencein the signal to noise ratio. More complex structures can provide notonly increased quantum efficiency but also good uniformity from pixel topixel. In addition, devices of the present disclosure exhibit increasedresponsivity as compared to traditional photosensitive devices. Forexample, in one aspect the responsivity can be greater than or equal to0.8 A/W for wavelengths greater than 1000 nm for semiconductor substratethat is less than 100 μm thick. In other embodiment the responsivity canbe greater than 0.5 A/W for wavelengths greater than 1100 nm forsemiconductor substrate that is less than 50 μm thick.

A variety of semiconductor materials are contemplated for use with thedevices and methods according to aspects of the present disclosure.Non-limiting examples of such semiconductor materials can include groupIV materials, compounds and alloys comprised of materials from groups IIand VI, compounds and alloys comprised of materials from groups III andV, and combinations thereof. More specifically, exemplary group IVmaterials can include silicon, carbon (e.g. diamond), germanium, andcombinations thereof. Various exemplary combinations of group IVmaterials can include silicon carbide (SiC) and silicon germanium(SiGe). In one specific aspect, the semiconductor material can be orinclude silicon. Exemplary silicon materials can include amorphoussilicon (a-Si), microcrystalline silicon, multicrystalline silicon, andmonocrystalline silicon, as well as other crystal types. In anotheraspect, the semiconductor material can include at least one of silicon,carbon, germanium, aluminum nitride, gallium nitride, indium galliumarsenide, aluminum gallium arsenide, and combinations thereof.

Exemplary combinations of group II-VI materials can include cadmiumselenide (CdSe), cadmium sulfide (CdS), cadmium telluride (CdTe), zincoxide (ZnO), zinc selenide (ZnSe), zinc sulfide (ZnS), zinc telluride(ZnTe), cadmium zinc telluride (CdZnTe, CZT), mercury cadmium telluride(HgCdTe), mercury zinc telluride (HgZnTe), mercury zinc selenide(HgZnSe), and combinations thereof.

Exemplary combinations of group III-V materials can include aluminumantimonide (AlSb), aluminum arsenide (AlAs), aluminum nitride (AlN),aluminum phosphide (AlP), boron nitride (BN), boron phosphide (BP),boron arsenide (BAs), gallium antimonide (GaSb), gallium arsenide(GaAs), gallium nitride (GaN), gallium phosphide (GaP), indiumantimonide (InSb), indium arsenide (InAs), indium nitride (InN), indiumphosphide (InP), aluminum gallium arsenide (AlGaAs, Al_(x)Ga_(1-x)As),indium gallium arsenide (InGaAs, In_(x)Ga_(1-x)As), indium galliumphosphide (InGaP), aluminum indium arsenide (AllInAs), aluminum indiumantimonide (AlInSb), gallium arsenide nitride (GaAsN), gallium arsenidephosphide (GaAsP), aluminum gallium nitride (AlGaN), aluminum galliumphosphide (AlGaP), indium gallium nitride (InGaN), indium arsenideantimonide (InAsSb), indium gallium antimonide (InGaSb), aluminumgallium indium phosphide (AlGaInP), aluminum gallium arsenide phosphide(AlGaAsP), indium gallium arsenide phosphide (InGaAsP), aluminum indiumarsenide phosphide (AlInAsP), aluminum gallium arsenide nitride(AlGaAsN), indium gallium arsenide nitride (InGaAsN), indium aluminumarsenide nitride (InAlAsN), gallium arsenide antimonide nitride(GaAsSbN), gallium indium nitride arsenide antimonide (GaInNAsSb),gallium indium arsenide antimonide phosphide (GaInAsSbP), andcombinations thereof.

The semiconductor substrate can be of any thickness that allowselectromagnetic radiation detection and conversion functionality, andthus any such thickness of semiconductor material is considered to bewithin the present scope. In some aspects, the textured region increasesthe efficiency of the device such that the semiconductor substrate canbe thinner than has previously been possible. Decreasing the thicknessof the semiconductor substrate reduces the amount of semiconductormaterial required to make such a device. In one aspect, for example, thesemiconductor substrate has a thickness of from about 500 nm to about 50μm. In another aspect, the semiconductor substrate has a thickness ofless than or equal to about 100 μm. In yet another aspect, thesemiconductor substrate has a thickness of from about 1 μm to about 10μm. In a further aspect, the semiconductor substrate can have athickness of from about 5 μm to about 50 μm. In yet a further aspect,the semiconductor substrate can have a thickness of from about 5 μm toabout 10 μm.

Additionally, various types of semiconductor materials are contemplated,and any such material that can be incorporated into an electromagneticradiation detection device is considered to be within the present scope.In one aspect, for example, the semiconductor material ismonocrystalline. In another aspect, the semiconductor material ismulticrystalline. In yet another aspect, the semiconductor material ismicrocrystalline. It is also contemplated that the semiconductormaterial can be amorphous. Specific nonlimiting examples includeamorphous silicon or amorphous selenium.

The semiconductor materials of the present disclosure can also be madeusing a variety of manufacturing processes. In some cases themanufacturing procedures can affect the efficiency of the device, andmay be taken into account in achieving a desired result. Exemplarymanufacturing processes can include Czochralski (Cz) processes, magneticCzochralski (mCz) processes, Float Zone (FZ) processes, epitaxial growthor deposition processes, and the like. It is contemplated that thesemiconductor materials used in the present invention can be acombination of monocrystalline material with epitaxially grown layersformed thereon.

A variety of dopant materials are contemplated for the formation of themultiple doped regions, and any such dopant that can be used in suchprocesses to surface modify a material is considered to be within thepresent scope. It should be noted that the particular dopant utilizedcan vary depending on the material being doped, as well as the intendeduse of the resulting material. For example, the selection of potentialdopants may differ depending on whether or not tuning of thephotosensitive device is desired.

A dopant can be either charge donating or accepting dopant species. Morespecifically, an electron donating or a hole donating species can causea region to become more positive or negative in polarity as compared tothe semiconductor substrate. In one aspect, for example, the dopedregion can be p-doped. In another aspect the doped region can ben-doped. A highly doped region can also be formed on or near the dopedregion to create a pinned diode. In one non-limiting example, thesemiconductor substrate can be negative in polarity, and a doped regionand a highly doped region can be doped with p+ and n dopantsrespectively. In some aspects, variations of n(−−), n(−), n(+), n(++),p(−−), p(−), p(+), or p(++) type doping of the regions can be used.

In one aspect, non-limiting examples of dopant materials can include S,F, B, P, N, As, Se, Te, Ge, Ar, Ga, In, Sb, and combinations thereof. Itshould be noted that the scope of dopant materials should include, notonly the dopant materials themselves, but also materials in forms thatdeliver such dopants (i.e. dopant carriers). For example, S dopantmaterials includes not only S, but also any material capable being usedto dope S into the target region, such as, for example, H₂S, SF₆, SO₂,and the like, including combinations thereof. In one specific aspect,the dopant can be S. Sulfur can be present at an ion dosage level ofbetween about 5×10¹⁴ ions/cm² and about 1×10¹⁶ ions/cm². Non-limitingexamples of fluorine-containing compounds can include ClF₃, PF₅, F₂ SF₆,BF₃, GeF₄, WF₆, SiF₄, HF, CF₄, CHF₃, CH₂F₂, CH₃F, C₂F₆, C₂HF₅, C₃F₈,C₄F₈, NF₃, and the like, including combinations thereof. Non-limitingexamples of boron-containing compounds can include B(CH₃)₃, BF₃, BCl₃,BN, C₂B₁₀H₁₂, borosilica, B₂H₆, and the like, including combinationsthereof. Non-limiting examples of phosphorous-containing compounds caninclude PF₅, PH₃, and the like, including combinations thereof.Non-limiting examples of chlorine-containing compounds can include Cl₂,SiH₂Cl₂, HCl, SiCl₄, and the like, including combinations thereof.Dopants can also include arsenic-containing compounds such as AsH₃ andthe like, as well as antimony-containing compounds. Additionally, dopantmaterials can include mixtures or combinations across dopant groups,i.e. a sulfur-containing compound mixed with a chlorine-containingcompound. In one aspect, the dopant material can have a density that isgreater than air. In one specific aspect, the dopant material caninclude Se, H₂S, SF₆, or mixtures thereof. In yet another specificaspect, the dopant can be SF₆ and can have a predetermined concentrationrange of about 5.0×10⁻⁸ mol/cm³ to about 5.0×10⁻⁴ mol/cm³. As onenon-limiting example, SF₆ gas is a good carrier for the incorporation ofsulfur into the semiconductor material via a laser process withoutsignificant adverse effects on the material. Additionally, it is notedthat dopants can also be liquid solutions of n-type or p-type dopantmaterials dissolved in a solution such as water, alcohol, or an acid orbasic solution. Dopants can also be solid materials applied as a powderor as a suspension dried onto the wafer.

As a further processing note, the semiconductor substrate can beannealed for a variety of reasons, including dopant activation,semiconductor damage repair, and the like. The semiconductor substratecan be annealed prior to texturing, following texturing, duringtexturing, or any combination thereof. Annealing can enhance thesemiconductive properties of the device, including increasing thephotoresponse properties of the semiconductor materials by reducing anyimperfections in the material. Additionally, annealing can reduce damagethat may occur during the texturing process. Although any known annealcan be beneficial and would be considered to be within the presentscope, annealing at lower temperatures can be particularly useful. Sucha “low temperature” anneal can greatly enhance the external quantumefficiency of devices utilizing such materials. In one aspect, forexample, the semiconductor substrate can be annealed to a temperature offrom about 300° C. to about 1100 C°. In another aspect, thesemiconductor substrate can be annealed to a temperature of from about500° C. to about 900° C. In yet another aspect, the semiconductorsubstrate can be annealed to a temperature of from about 700° C. toabout 800° C. In a further aspect, the semiconductor substrate can beannealed to a temperature that is less than or equal to about 850° C.

The duration of the annealing procedure can vary according to thespecific type of anneal being performed, as well as according to thematerials being used. For example, rapid annealing processes can beused, and as such, the duration of the anneal may be shorter as comparedto other techniques. Various rapid thermal anneal techniques are known,all of which should be considered to be within the present scope. In oneaspect, the semiconductor substrate can be annealed by a rapid annealingprocess for a duration of greater than or equal to about 1 μs. Inanother aspect, the duration of the rapid annealing process can be fromabout 1 μs to about 1 ms. As another example, a baking or furnace annealprocess can be used having durations that may be longer compared to arapid anneal. In one aspect, for example, the semiconductor substratecan be annealed by a baking anneal process for a duration of greaterthan or equal to about 1 ms to several hours.

Various types of dielectric region configurations are contemplated, andany configuration that can be incorporated into a photosensitive deviceis considered to be within the present scope. One benefit to such adielectric region pertains to the isolation provided between thetextured region and the doped regions that form the junction. In oneaspect, for example, the dielectric region can be positioned tophysically isolate the textured region from the junction. In this way,the creation of the textured region can be isolated from the dopedregions, thus precluding undesirable effects of the texturing processfrom affecting the junction. In another aspect, the dielectric regioncan be a dielectric material, and thus the dielectric region could beused to electrically isolate the textured region from the junction. Insome cases, the dielectric region is coupled directly to at least one ofthe doped regions forming the junction.

The dielectric region can be made from a variety of materials, and suchmaterials can vary depending on the device design and desiredcharacteristics. Non-limiting examples of such materials can includeoxides, nitrides, oxynitrides, and the like, including combinationsthereof. In one specific aspect, the dielectric region includes anoxide. Additionally, the dielectric region can be of variousthicknesses. In one aspect, for example, the dielectric region has athickness of from about 100 nm to about 1 micron. In another aspect, thedielectric region has a thickness of from about 5 nm to about 100 nm. Inyet another aspect, the dielectric region has a thickness of from about20 nm to about 50 nm. It should be noted that, in cases where thetextured region is a portion of the dielectric region (e.g. a dielectriclayer) that has been textured, the thickness of the dielectric materialwould be increased to account for the texturing. Thus the thicknessranges for the dielectric region provided here would be measured as thethickness of the dielectric region not including the textured portion.

The textured region can function to diffuse electromagnetic radiation,to redirect electromagnetic radiation, to absorb electromagneticradiation, and the like, thus increasing the quantum efficiency of thedevice. In the present BSI devices, electromagnetic radiation passingthrough the semiconductor substrate can contact the textured region. Thetextured region can include surface features to thus increase theeffective absorption length of the photosensitive pixel. Such surfacefeatures can be micron-sized and/or nano-sized, and can be any shape orconfigurations. Non-limiting examples of such shapes and configurationsinclude cones, pillars, pyramids, micolenses, quantum dots, invertedfeatures, gratings, protrusions, scallops, and the like, includingcombinations thereof. Additionally, factors such as manipulating thefeature sizes, dimensions, material type, dopant profiles, texturelocation, etc. can allow the diffusing region to be tunable for aspecific wavelength. In one aspect, tuning the device can allow specificwavelengths or ranges of wavelengths to be absorbed. In another aspect,tuning the device can allow specific wavelengths or ranges ofwavelengths to be reduced or eliminated via filtering.

Tuning can also be accomplished through the relative location of thetexture region within the device, modifying the dopant profile(s) ofregions within the device, dopant selection, and the like. Additionally,material composition near the textured region can create a wavelengthspecific photosensing pixel device. It should be noted that a wavelengthspecific photosensing pixel can differ from one pixel to the next, andcan be incorporated into an imaging array. For example a 4×4 array caninclude a blue pixel, a green pixel, a red pixel, and infrared lightabsorbing pixel, or a blue pixel, two green pixels, and a red pixel.

The textured regions can also be made to be selective to polarized lightand light of particular polarizations. In one specific example, if thetextured region includes a one dimensional grating of grooves on a highindex of refraction material then the scattering of the light willdepend upon the polarization of the light and the pixels can selectlight of specific linear polarizations.

Textured regions according to aspects of the present disclosure canallow a photosensitive device to experience multiple passes of incidentelectromagnetic radiation within the device, particularly at longerwavelengths (i.e. infrared). Such internal reflection increases theeffective absorption length to be greater than the thickness of thesemiconductor substrate. This increase in absorption length increasesthe quantum efficiency of the device, leading to an improved signal tonoise ratio.

The materials used for producing the textured region can vary dependingon the design and the desired characteristics of the device. As such,any material that can be utilized in the construction of a texturedregion is considered to be within the present scope. Non-limitingexamples of such materials include semiconductor materials, dielectricmaterials, conductive materials (e.g. metals), silicon, polysilicon,amorphous silicon, transparent conductive oxides, and the like,including composites and combinations thereof. In one specific aspect,the textured layer can be a textured polysilicon layer. Thus apolysilicon layer can be deposited onto the dielectric region and thentextured to form the textured region. In another aspect, the texturedlayer can be a textured dielectric layer. In this case the texturedregion can be a portion of the dielectric layer making up the dielectricregion. In yet another aspect the textured layer can be a transparentconductive oxide or another semiconductor material. In the case ofdielectric layers, the textured region can be a textured portion of thedielectric region or the textured region can be formed from otherdielectric material deposited over the dielectric region. In the case ofsemiconductor materials, forming the textured region can includedepositing a semiconductor material on the dielectric region andtexturing the semiconductor material to form the textured region. Inanother aspect, the semiconductor material can be bonded or adhered tothe dielectric region. The texturing process can texture the entiresemiconductor material or only a portion of the semiconductor material.In one specific aspect, a polysilicon layer can be deposited over thedielectric region and textured and patterned by an appropriate technique(e.g. a porous silicon etch) to form the textured region. In yet anotheraspect, a polysilicon layer can be deposited over the dielectric regionand textured and patterned by using a mask, photolithography, and anetch to define a specific structure or pattern.

In addition to surface features, the textured region can have a surfacemorphology that is designed to focus or otherwise direct electromagneticradiation, thus enhancing the quantum efficiency of the device. Forexample, in one aspect the textured region has a surface morphologyoperable to direct electromagnetic radiation into the semiconductorsubstrate. Non-limiting examples of various surface morphologies includesloping, pyramidal, inverted pyramidal, spherical, square, rectangular,parabolic, ellipsoidal, asymmetric, symmetric, scallops, gratings,pillars, cones, microlenses, quantum dots, and the like, includingcombinations thereof.

One example of such a surface morphology is shown in FIG. 4. Withoutintending to be limited to any operational theory, the followingdescription provides one possible explanation for the effects of surfacemorphology in tuning a textured region for specific wavelengths ofelectromagnetic radiation. FIG. 4 shows a textured device 400 having asurface morphology that affects the near infrared wavelength response. Asemiconductor substrate 402 is shown having an illuminated surface 404to receive incident electromagnetic radiation 406. The semiconductorsubstrate 402 further has a textured region 408 (e.g. dielectric)coupled thereto at a surface that is opposite to the illuminated surface404. The textured region 408 has a surface morphology configured in anundulating pattern 410 with grooves, ridges, or similar patterns toproduce an internal reflection that is not specular. In the nearinfrared, the index of refraction of silicon is about η=3.42 and thereflectance is about R=30% from a single planar surface, andtransmittance through a single planar surface is T=70% for normallyincident waves. The absorption coefficient of silicon is very low in thenear infrared. Electromagnetic radiation under normal incidence,represented by arrow 412, is reflected from the illuminated surface 404,and this is shown as arrow 414. There are successive reflections fromboth the illuminated surface and the opposing side, represented byarrows 416 and 418, and internal reflections from the illuminatedsurface 404, represented by arrow 420, resulting in a total internalreflection. If there is neither a reflective metal layer 422 nortextured region, the total transmittance, To, is as shown in Equation(I) as:

T _(tot)=(TT)(1+R ² +R ⁴+ . . . )=(TT)/(1−R ²)  (I)

This result has been obtained using the sum of a geometric series. Ifboth surfaces are just polished silicon-air, then the totaltransmittance is 54% and the reflectance is 46%.

If the increase in the individual path lengths caused by the diffusescattering is neglected and if the absorption coefficient is very lowthen the total effective path length is determined by just the number ofreflections, and the total absorptance can be as shown in Equation (II):

A=αd(1+R ₂)(1+R ₁ R ₂ +R ₂ ² R ₂ ²+ . . . )=αd(1+R ₂)/(1−R ₁ R ₂)  (II)

Here, α is the absorption coefficient in reciprocal cm, d is thethickness of the sample in cm, and the effective increase in path lengthis Enh=(1+R₂)/(1−R₁ R₂). The internal quantum efficiency (IQE) in theinfrared where the absorption in silicon is low is thus IQE=αd(Enh). Theexternal quantum efficiency (EQE) is EQE=T₁IQE and EQE=T₁ αd(Enh).

If both sides of an infrared photo detector are polished then T₁=T₂=0.70and R₁=R₂=0.3, which gives Enh=1.4, IQE=1.4αd, and EQE=αd. If one sideis polished and the other side has an oxide and a metal reflector, thenR₁=0.3 and R₂=1.0, yielding an enhancement in infrared absorptance orEnh=3. T₁ is the transmittance of radiation incident on the firstsurface. T₂ is the transmittance of radiation striking the secondsurface from the semiconductor side. R₁ is the amount of radiationreflected back into the semiconductor for radiation striking the firstsurface from the semiconductor side. R₂ is the amount of radiationreflected back into the semiconductor for radiation striking the secondsurface from the semiconductor side.

In one aspect that can improve the infrared response, the illuminatedside 404 is polished but the opposing side 410 is a textured dielectricmaterial 408 with a reflecting region 422 (see FIG. 4). The texturingcan be realized in a fashion to produce a true diffuse scattering (i.e.a Lambertian scattering) at the infrared wavelengths. This diffusescattering layer/reflecting layer combination, in essence yields anR₂=100%, which is a diffuse reflector. The reflectance of the polishedfront side to the scattered light radiation is determined by solid angleconsiderations. Any incident light with an angle of incidence greaterthan the critical angle θ_(c), 424, will be totally internally reflected420. If the backside scattering is totally diffused in a half sphere orLambertian, the transmittance is then determined by the area of thesurface (πr²) within the critical angle θ_(c), 424, in this case 170 forsilicon and air. The radius of the circle is r=d·sin(17°), where d isthe thickness of the sample. This area is divided by the area of thehalf sphere (2πd²). If the backside scattering is totally diffuse, thetransmittance of the front planar surface is then roughly T₁=3% and thereflectance R₁=97%. The path length enhancement factor can be verylarge, as is shown in Equation (III):

Enh=(1+R ₂)/(1−R ₁ R ₂)=66  (III)

This would result in an IQE=66αd and an EQE=46. If the backside includesa textured region and a truly diffusive scattering surface, and amirror-like surface is used behind the back side, a very largeenhancement of absorptance in the near infrared can be achieved. If theabsorption in the semiconductor substrate is not assumed to be small butrather is taken into account, it can be shown that the enhancementfactor for the IQE due to multiple reflections is modified from Equation(I) and as is shown in Equation (IV):

Enh=(1−exp(−αd))(1+R ₂exp(−αd))/(1−R ₁ R ₂exp(−2αd))  (IV)

This allows a calculation of the responsivity in terms of the electricalcurrent in Amperes per incident light power in Watts of photo detectorsof different thickness d for different wavelengths λ, since theabsorption coefficient α(λ) is a function of wavelength (see FIG. 5). Ifit is assumed that the textured side is an ideal reflector (i.e. R₂=1.0)and the amount of diffusive scattering of the textured surface variesfrom that of a planar surface, then the fraction of light reflected backfrom the opposing illuminated surface may vary. If the textured surfaceis planar, then there is only specular reflection, and R₁=0.3 and theenhancement in responsivity, as indicated by curve 600, is not large. Ifthe textured surface is an ideal Lambertian diffusive surface then thefraction of light reflected back from the front surface may be verylarge, R₁=0.97. Several values of R₁ as illustrated by curves, 601, 602,and 603 are discussed herein and illustrated in FIG. 5, for a diffusereflector, and these represent the fraction of light internallyreflected back at the front surface. For purposes of the presentdisclosure, values of R₁≧0.9 (curve 601) are deemed useful. Theenhancement in absorptance described by Equation (IV) then varies withthe fraction of light radiation reflected back from the illuminatedsurface and thickness of the sample, as is illustrated in FIG. 5. Itshould be noted that, while the techniques described herein have beenused to enhance the absorptance at infrared and red light radiation,they are also applicable to visible light as the thickness of thesilicon layer becomes thinner. Scattering and multiple internalreflections can also be used to increase the absorptance at yellow,green and even blue light that will not be totally absorbed in singlepasses within thin silicon layers. These techniques can be applied thento visible imagers with thin silicon absorption layers.

In another aspect, FIG. 6 shows a cross-section of a BSI photosensitivepixel device 600. The photosensitive pixel device can include asemiconductor substrate 602 that can be referred to as bulksemiconductor material. The semiconductor substrate 602 includes atleast one doped region 604, 605, that can be doped with a chargedonating species, such as an electron donating or hole donating species,to cause the region to become more n-type or p-type as compared to thesemiconductor substrate. In one aspect, for example, the doped regioncan be p-doped. In another aspect the doped region can be n-doped. Thedevice can further include various metal regions 606, at least one via608, a dielectric region 610, isolation element(s) 612, such as trenchisolation elements, and an electrical transfer element 614.

Trench isolation elements can maintain pixel to pixel uniformity byreducing optical and electrical crosstalk. The isolation elements can beshallow or deep trench isolation. The trench isolation elements of FIG.6 are shown as deep trench isolation elements. The isolation elementscan include various materials, including, without limitation, dielectricmaterials, reflective materials, conductive materials, light diffusingfeatures, and the like. These isolation regions can be configured toreflect incident light until it is absorbed, thereby increase theeffective absorption length of the device.

A lens 624 and an anti-reflective coating 616 can be disposed on thebackside of the pixel following thinning and trench isolation. A colorfilter 618 can be optically coupled to the lens to allow specificwavelength filtering of the electromagnetic radiation. A textured region620 can be coupled to the semiconductor substrate 602 opposite the lens624 in order to provide diffusive scattering and reflection of theincident electromagnetic radiation that passes through to the front sideof the pixel. Thus the electromagnetic radiation can be focused withinthe semiconductor substrate 602 to the combined action of the texturedregion 620 and the isolation features 612. Additionally, side wallinsulators 622 can be formed about the transfer element 606 and alsoaround textured region 620 to facilitate proper spacing.

In general, isolation features can maintain pixel to pixel uniformitywhen multiple pixels are used in association by reducing optical andelectrical crosstalk there between. The isolation feature can be shallowor deep, depending on the desired design. The isolation features can begenerated using various materials including, without limitation,dielectric materials, reflective materials, conductive materials, lightdiffusing features, and the like. Additionally, in some aspects theisolation feature can be a void in the semiconductor substrate. In oneaspect, isolation features can also be configured to reflect incidentelectromagnetic radiation until it is absorbed, thereby increasing theeffective absorption length of the incident light and reducing opticalcrosstalk into neighboring devices. Furthermore, the devices accordingto aspects of the present disclosure can also independently include oneor more vias, dielectric regions, and the like.

In other aspects of the present disclosure, various methods of making aphotosensitive device as described herein are contemplated. In oneaspect, as is shown in FIG. 7, a method of making a backside-illuminatedphotosensitive imager device can include traditional fabrication stepsalong with additional steps of forming isolation features 702, doping aportion of the semiconductor material to form n+ implant 704, doping aportion with an n+ implant to form a source-drain 706, doping a portionof the semiconductor substrate with a p+ dopant to form p+ implant 708,and growing and etching a gate oxide region 710. The method can alsoinclude depositing and etching to form a polysilicon gate 712, forming atextured surface near the p+ implant region 714, and depositing thecontacts 716. It should be noted that the present scope is not limitedto the above described doping scheme. Thus, this process can be appliedin modular form to the steps involved in the fabrication of aconventional BSI CMOS imager pixel arrays. In other words, this modularprocessing method can be an insert of a process module in the processflow of the fabrication of a conventional BSI imager. In one aspect, theconventional process can be stopped just before the first via etch butafter the deposition of the interlayer dielectric. At this point, ratherthan etch a via, the mask that was used to define the p-typeimplantation on top of the n-type diode can be used to mask and open thearea over the p-type layer. A sidewall spacer process can be employed onthe inside border of this opening to insure adequate spacing of thetextured layers to be deposited away from the transfer device. Adielectric region, a textured region, and in some cases a metal or oxidereflector can be deposited in the area over the p-type region. As such,this module can be introduced into a conventional process flow withminimal changes. Subsequently, an interlayer dielectric similar to thefirst interlayer dielectric can be deposited over the metal reflector,and the combined interlayer dielectrics can be chemically andmechanically polished to planarize the surface. The normal contact maskcan then be used to open vias resulting in minimal changes to theconventional process flow. A via can be opened to the metal reflectorlayer to control the potential of this layer and if appropriate, thetextured layer to avoid charging during processing or for the region,can act as an MOS gate.

CMOS imagers are commonly used as visible light cell phone, still frameand video cameras, with BSI devices becoming a common structure. Camerasfor use as infrared imagers for security and surveillance are, however,not commonly available. The present methods allow the conversion ofvisible light imagers to include the capability for imaging in the nearinfrared. Thus, the texture processing methods described can be adaptedto a traditional fabrication process for visible imagers to provide anadditional capability for near infrared imaging. The CMOS imagers canemploy any pixel design including more than one transistor or transferdevice. In some aspects, the transfer device can be a p-channel transfergate or an n-channel transfer gate and associated implant polarities.

Conventional CCD imagers employ photodiode detectors similar to thoseused in CMOS imagers, and as such, the present methods can also beapplied to the fabrication of CCD imagers to form devices capable ofdetecting infrared wavelengths in appreciable amounts and withappreciable enhancement.

The photodetecting diode used in the imager pixel can also include MOSdiodes rather than doped junction diodes. MOS diodes use a pulsed biasto form a depletion region and light collecting region near the surfacerather than a junction diode. The method described herein can equally beapplied and inserted into imager pixels with MOS diodes or CCD imagerswhere the light generated carriers are collected in a surface depletionregion. In these latter cases the metal reflecting gate can be drivenwith a voltage to act as a MOS or CCD gate.

In one aspect, as is shown in FIG. 8a , a BSI photosensitive imagerdevice 800, can include a textured region 802, a reflecting layer 804,and a dielectric layer 806 disposed between the semiconductor substrate808 and the textured region 802. The device can further include dopedregions 810, 812, a drain region 814, sidewall spacers 816, and atransfer device having a gate insulator 818 and a polysilicon gate 820,as has been described. The reflecting/diffusing region 822, shownseparately in FIG. 8b , is thus effectively a process module that can beinserted into a standard CMOS imager manufacturing process as has beendescribed. In one aspect, this process module of forming thereflective/diffusing region can be inserted after deposition of thefirst dielectric spacer layer and before a contact etch step in a CMOSimage sensor manufacturing process. A mask can be used to create theimplantation of the P+ surface region on the photodiode, which can alsobe used to etch an aperture in the newly formed dielectric layer. Thespacers 816 can be formed on the inside of this hole using conventionaltechniques to space the textured region 802 away from the edge of the P+region. The hole can then be filled with any one of a variety of“process modules” to form the textured region 802. After completion ofthe textured region, the conventional process can be continued startingwith the contact etch mask. It should be noted that the present scopealso includes aspects having a lens without an anti-reflecting coating,and aspects having an anti-reflecting coating associated with thesemiconductor substrate without a lens. Additionally, a color filter(not shown) can be optically coupled to the lens to allow specificwavelength filtering of the electromagnetic radiation. Incidentelectromagnetic radiation passing through the color filter prior tocontacting the semiconductor substrate can be filtered according to thecharacteristics of the filter.

One simple aspect to enhance the infrared response of photo detectingpixels can include a module consisting essentially of a dielectricregion, an oxide, and a metal reflector region without texture. Eventhis simple structure can provide, by virtue of Equation III withR₂=1.0, R₁=0.3, an enhancement in the infrared response over that of asingle pass of radiation by a factor of about 2.

The textured region, including surface features as well as surfacemorphologies, can be formed by various techniques, including plasmaetching, reactive ion etching, porous silicon etching, lasing, chemicaletching (e.g. anisotropic etching, isotropic etching), nanoimprinting,material deposition, selective epitaxial growth, and the like. In oneaspect, the texturing process can be performed during the manufacture ofthe photosensitive device. In another aspect, the texturing process canbe performed on a photosensitive device that has previously been made.For example, a CMOS, CCD, or other photosensitive element can betextured following manufacture. In this case, material layers may beremoved from the photosensitive element to expose the semiconductorsubstrate or the dielectric region upon which a textured region can beformed.

Additionally, it is contemplated that the textured region can be formedhaving surface features with a size and position distribution thatallows tuning to a desired light wavelength or range of wavelengths. Assuch, a given textured region can contain an arrangement of surfacefeatures that facilitate tuning of the textured region to a preselectedwavelength of light. Any wavelength that can be selectively tuned viasurface features in the textured region is considered to be within thepresent scope. In one aspect, for example, the preselected wavelength orwavelengths of light can be in the near infrared or infrared range. Inanother aspect, the preselected wavelength of light can be greater thanor equal to about 800 nm.

In one aspect, tuning can be accomplished by generating a texture withsufficient long-range lateral order such that interference directs thephotons within a certain wavelength range in such a direction that theywill experience total internal reflection (TIR) on the surface oppositethe texture. This will enhance absorptance and QE. In another aspect,the texture will be additionally tuned to minimize photons reflectedinto the range of angles where TIR will not occur. This further enhancesabsorptance and QE. In another aspect, the texture will be additionallytuned to keep the angle at which the photons at the preselectedwavelength impinge on the surface opposite the texture as close tonormal as possible while still maintaining TIR. This maximized thepropagation path, thereby further increasing absorptance and QE, andsimultaneously minimizing the optical crosstalk at that preselectedwavelength.

For texture with poor long-range lateral order, the texture can be tunedfor wavelength selection. In one aspect, the average lateral modulationspatial frequency will be large enough compared to the diffractionlimited spot size at the preselected wavelength that effective indexdescriptions are inaccurate and scattering is substantial. In anotheraspect, the average modulation amplitude will not be much smaller thanthe preselected wavelength, so that the surface will not substantiallybehave as though it were planar.

Regardless of the degree of long-range lateral order, the individualfeatures that make up the texture can be adjusted in shape so as tomaximize the number of photons that are incident on the opposite surfaceat an angle beyond the critical angle, θ_(c). In another aspect, theseindividual features are adjusted to minimize the number of photons thatare incident on the opposite surface at an angle less than the criticalangle, θ_(c).

The texture can also be tuned to provide polarization selectivity. Inone aspect, the rotational symmetry in the plane of the texture can bemaximized, thereby leading to uniform behavior over the maximumpolarization states. In another aspect, the rotational symmetry in theplane of the texture can be minimized, thereby leading to maximaldifference in the behavior for differing polarization states.

One effective method of producing a textured region is through laserprocessing. Such laser processing allows discrete locations of thedielectric region or other substrate to be textured. A variety oftechniques of laser processing to form a textured region arecontemplated, and any technique capable of forming such a region shouldbe considered to be within the present scope. Laser treatment orprocessing can allow, among other things, enhanced absorptanceproperties and thus increased electromagnetic radiation focusing anddetection. The laser treated region can be associated with the surfacenearest the impinging electromagnetic radiation or, in the case of BSIdevices, the laser treated surface can be associated with a surfaceopposite in relation to impinging electromagnetic radiation, therebyallowing the radiation to pass through the semiconductor substratebefore it hits the laser treated region.

In one aspect, for example, a target region of the semiconductormaterial can be irradiated with laser radiation to form a texturedregion. Examples of such processing have been described in furtherdetail in U.S. Pat. Nos. 7,057,256, 7,354,792 and 7,442,629, which areincorporated herein by reference in their entireties. Briefly, a surfaceof a substrate material is irradiated with laser radiation to form atextured or surface modified region. Such laser processing can occurwith or without a dopant material. In those aspects whereby a dopant isused, the laser can be directed through a dopant carrier and onto thesubstrate surface. In this way, dopant from the dopant carrier isintroduced into the target region of the substrate material. Such aregion incorporated into a substrate material can have various benefitsin accordance with aspects of the present disclosure. For example, thetarget region typically has a textured surface that increases thesurface area of the laser treated region and increases the probabilityof radiation absorption via the mechanisms described herein. In oneaspect, such a target region is a substantially textured surfaceincluding micron-sized and/or nano-sized surface features that have beengenerated by the laser texturing. In another aspect, irradiating thesurface of the substrate material includes exposing the laser radiationto a dopant such that irradiation incorporates the dopant into thesubstrate. Various dopant materials are known in the art, and arediscussed in more detail herein.

Thus the surface of the substrate or dielectric region is chemicallyand/or structurally altered by the laser treatment, which may, in someaspects, result in the formation of surface features appearing asmicrostructures or patterned areas on the surface and, if a dopant isused, the incorporation of such dopants into the substrate material. Insome aspects, the features or microstructures can be on the order of 50nm to 20 μm in size (i.e. size at the base, or in some casescenter-to-center) and can assist in the absorption of electromagneticradiation. In other words, the textured surface can increase theprobability of incident radiation being absorbed.

The type of laser radiation used to surface modify a material can varydepending on the material and the intended modification. Any laserradiation known in the art can be used with the devices and methods ofthe present disclosure. There are a number of laser characteristics,however, that can affect the surface modification process and/or theresulting product including, but not limited to the wavelength of thelaser radiation, pulse duration, pulse fluence, pulsing frequency,polarization, laser propagation direction relative to the semiconductormaterial, etc. In one aspect, a laser can be configured to providepulsatile lasing of a material. A short-pulsed laser is one capable ofproducing femtosecond, picosecond and/or nanosecond pulse durations.Laser pulses can have a central wavelength in a range of about fromabout 10 nm to about 8 μm, and more specifically from about 200 nm toabout 1200 nm. The pulse duration of the laser radiation can be in arange of from about tens of femtoseconds to about hundreds ofnanoseconds. In one aspect, laser pulse durations can be in the range offrom about 50 femtoseconds to about 50 picoseconds. In another aspect,laser pulse durations can be in the range of from about 50 picosecondsto 100 nanoseconds. In another aspect, laser pulse durations are in therange of from about 50 to 500 femtoseconds.

The number of laser pulses irradiating a target region can be in a rangeof from about 1 to about 2000. In one aspect, the number of laser pulsesirradiating a target region can be from about 2 to about 1000. Further,the repetition rate or pulsing frequency can be selected to be in arange of from about 10 Hz to about 10 μHz, or in a range of from about 1kHz to about 1 MHz, or in a range from about 10 Hz to about 1 kHz.Moreover, the fluence of each laser pulse can be in a range of fromabout 1 kJ/m² to about 20 kJ/m², or in a range of from about 3 kJ/m² toabout 8 kJ/m².

As has been described, the devices according to aspects of the presentdisclosure can additionally include one or more reflecting regions. Thereflecting region can be deposited over the entire textured region oronly over a portion of the textured region. In some aspects, thereflecting region can be deposited over a larger area of the device thanthe textured region. The reflecting region can be positioned to reflectelectromagnetic radiation passing through the texture region backthrough the textured region. In other words, as electromagneticradiation passes into the semiconductor substrate, a portion that is notabsorbed contacts the textured region. Of that portion that contacts thetextured region, a smaller portion may pass though the textured regionto strike the reflecting region and be reflected back through thetextured region toward the semiconductor substrate.

A variety of reflective materials can be utilized in constructing thereflecting region, and any such material capable of incorporation into aphotosensitive device is considered to be within the present scope.Non-limiting examples of such materials include a Bragg reflector, ametal reflector, a metal reflector over a dielectric material, atransparent conductive oxide such as zinc oxide, indium oxide, or tinoxide, and the like, including combinations thereof. Non-limitingexamples of metal reflector materials can include silver, aluminum,gold, platinum, reflective metal nitrides, reflective metal oxides, andthe like, including combinations thereof. In one aspect, a BSIphotosensitive imager device can include a dielectric layer positionedbetween the reflecting region and the textured region. In one specificaspect, the dielectric layer can include an oxide layer and thereflecting region can include a metal layer. The surface of the metallayer on an oxide acts as a mirror-like reflector for the incidentelectromagnetic radiation from the backside. It should be noted that thereflective region is not biased with a voltage.

In another aspect, the textured region can include a hemisphericalgrained polysilicon or coarse grained polysilicon material and thereflective region can include a metal layer. The hemispherical grainedor coarse grained silicon can act as a diffusive scattering site for theincident optical radiation and the dielectric layer and the reflectiveregion together can act as a reflector.

In still another aspect, the photosensitive imager can include selectiveepitaxial silicon growth for generating the textured region on top ofthe junction formed by the doped regions (e.g. a photodiode) without thedielectric region being present (not shown). An oxide and metalreflector, for example, can be coupled to the textured region. Theepitaxial growth places the textured region away from the top of thejunction, and the rapid etch characteristics of grain boundaries can beused to create texturing.

Additionally, the textured surface of a metal on a roughened oxide canact as a diffusive scattering site for the incident electromagneticradiation and also as a mirror-like reflector. Other aspects can utilizeporous materials for the texturing. Porous polysilicon, for example, canbe oxidized or oxide deposited and a reflective region such as a metalreflector can be associated therewith to provide a scattering andreflecting surface. In another aspect, aluminum can be subjected toanodic oxidation to provide porous aluminum oxide, a high dielectricconstant insulator. This insulator can be coated with aluminum or othermetals to provide a scattering and reflecting surface.

In one specific aspect, a reflective region can include a transparentconductive oxide, an oxide, and a metal layer. The transparent oxide canbe textured and a metal reflector deposited thereupon. The texturedsurface of the metal on a roughened transparent conductive oxide can actas a diffusive scattering site for the incident electromagneticradiation.

In another specific aspect, a Bragg reflector can be utilized as areflective region. A Bragg reflector is a structure formed from multiplelayers of alternating materials with varying refractive indexes, or bysome other method of inducing a periodic variation in the propagationconstant. Each layer boundary causes a partial reflection of an opticalwave. For waves whose wavelength is close to four times the opticalthickness of the layers, the many reflections combine with constructiveinterference, and the layers act as a high-quality reflector. Thus thecoherent super-positioning of reflected and transmitted light frommultiple interfaces in the structure interfere so as to provide thedesired reflective, transmissive, and absorptive behavior. In oneaspect, the Bragg reflector layers can be alternating layers of silicondioxide and silicon. Because of the high refractive index differencebetween silicon and silicon dioxide, and the thickness of these layers,this structure can be fairly low loss even in regions where bulk siliconabsorbs appreciably. Additionally, because of the large refractive indexdifference, the optical thickness of the entire layer set can bethinner, resulting in a broader-band behavior and fewer fabricationssteps.

In another aspect, texturing can be applied to the light-incidentsurface of the semiconductor substrate in order to facilitate additionalscattering as light enters the device. In some aspects it can be usefulto also include trench isolation to preclude optical crosstalk betweenpixels due to this forward scattering. By also texturing the trenchisolation, light can be reflected back into the semiconductor from theedges of the pixel. It is noted that, for a BSI architecture, thelight-incident surface is on the back side of the semiconductorsubstrate opposite the photodiode.

In another embodiment of the present disclosure, a backside texturesurface an imager pixel is shown in FIGS. 9a,b . Specifically, FIG. 9ashows a top view and FIG. 9b shows a cross sectional view of a texturedregion 900 including pillars etched into a semiconductor material. Inone aspect, the textured region can include a plurality of pillarshaving a uniform size. In another aspect, the textured region caninclude a plurality of pillars in a regular (ordered pattern) array. Inyet another aspect, the textured region can include a plurality ofpillars of variable sizes (902, 904 in FIG. 9). In a further aspect, thetextured region can be arranged in a non-uniform array (see FIG. 9).Similarly, holes of uniform or variable sizes can be etched into thesilicon and covered by a thin layer of oxide and metal to achievesimilar effects as the pillars. Moreover, the pillar or hole featurescan be etched into a thin layer of oxide and metal, or they can beetched into a thicker oxide on the semiconductor substrate and coveredby a reflective material, such as silver or aluminum. In the former casethe minimum oxide thickness between the semiconductor substrate andmetal can be in the range of about 5 nm to 100 nm. In another aspect,the thickness can be about 40 nm.

The height of the pillars, shown in the cross sectional view of FIG. 9b, can vary depending on the design and desired use of the device. In oneaspect, however, the average pillar height can be a multiple of aquarter wavelength of the desired light wavelength selectivity in themedium to which they are etched (e.g., the semiconductor material or thedielectric material. In another aspect, the average center-to-centerdistance between pillars can be about one half the wavelength of lightdesired to be absorbed (½ λ), multiples of one half the wavelength, orlarger than half the wavelength of desired light selectivity. Thepillars can also be designed to be anti-reflecting to the specularreflection of light or the zero^(th) order diffraction from the texture.The higher order diffractions of light from the texture can be designedto optimize the reflection of light at any particular oblique angle. Inthis manner most of the incident light can be reflected at angles tomaximize light trapping in the imager pixel. Light scattered by thefront side textured region and striking the semiconductor side of theback interface can be totally internally reflected if it is outside ofthe critical angle as defined by Snell's law.

One exemplary method of creating a textured region such as a diffractiongrating is shown in FIGS. 10a-c . In this case, a semiconductorsubstrate 1002 having an oxide layer 1004 is provided. A mask 1006 as a2-dimensional grating is formed on the oxide layer 1004, as is shown inFIG. 10a . The oxide layer 1004 can then be etched with an isotropicetch 1008 to form a plurality of pillars 1010, as shown in FIG. 10b . Inone aspect, undercutting of the pillars by the isotropic etch 1008 canresults in pillars 1010 that are narrow at the top of the pillar thanthe bottom. The mask 1006 can be removed and a rinse etch used to roundoff sharp corners and tops of the pillars 1010, as is shown in FIG. 10c. A reflecting material 1012 (e.g. a metal) can be deposited as areflector. The rounded surfaces can reduce the amplitude of the specularreflection or zero^(th) order diffraction of light from the diffractiongrating (i.e., the textured region). Thus, the light will be scatteredmore effectively at oblique angles. FIG. 10c also shows the thickness(t_(ox)) 1014 of oxide layer 1004, the grating thickness (t_(gr)) 1016,and the distance from pillar peak-to-peak (T) 1018. While the dimensionsof the grating can vary depending on the design of the device and thedesired usage, in one aspect the average thickness of t_(ox) can have arange of from about 10 nm to about 100 nm. In another aspect, the to,can have a thickness of less than about 50 nm. The average gratingthickness t_(gr), measured from the base of the oxide layer 1004 to apillar peak 1010, can be about a quarter of the wavelength of thedesired light wavelength selectivity in the oxide layer (˜¼λ). Forexample, light having a wavelength of about 1000 nm can dictate t_(gr)having a thickness of greater than about 175 nm. The average distance(T) from pillar peak-to-peak can be greater than 300 nm for light havinga wavelength of greater than 1000 nm.

Another exemplary method of creating a textured region such as adiffraction grating is shown in FIGS. 11a-d , which are cross-sectionalviews of a photosensitive device at different stages of manufacturing.In one specific aspect, the semiconductor material can be amorphoussilicon, which can have a texture surface with a high index ofrefraction because amorphous silicon can be deposited at lowtemperatures.

Specifically, FIGS. 11a-d show a one dimensional grating, such as aplurality of grooves, or in two dimensions an array of pyramidalstructures being formed on or in the photosensitive device. In FIG. 11a, a semiconductor material 1102 can be coupled to a thin layer of oxide1104 and a layer of nitride 1106. In this aspect, the front side 1102 ofthe pixel in the P+ diode surface area has been opened in the firstinterlayer dielectric. The semiconductor material 1102 can be etched1108 to form a mask 1110 using standard techniques on an opposite sidewhere incident light enters the device. Furthermore, in some aspectsanother layer of a semiconductor material 1112 such as amorphous siliconcan be deposited, and an anisotropic etch can be used in a “sidewall”like process. A reflecting region comprising a dielectric layer 1114 andmetallic layer 1116 can be deposited on the etched surface. In onedimensional arrays this produces a series lines of material 1118 andgrooves in the semiconductor material that functions as a diffractiongrating 1120 (FIG. 11c ). The distance from one groove to theneighboring groove (2k, FIG. 11b ) can be determined by the diffractionangle for different orders (m) of diffraction. The first orderdiffraction, 1122, can be selected to be a large angle βeta (β) 1124,thereby maximizing light trapping effects. The height of the originalpillars are measured from the valley to the peak of the pillaridentified as h, which determines the “blaze” angle gamma (γ) 1126 ofthe grating and the efficiency of the diffraction into differentdiffraction orders. In two dimensions, the semiconductor structure canhave pillars rather than lines and the final structure is a twodimensional grating. Similar one dimensional grating designconsiderations can also be applied.

In yet another aspect, fully sub-wavelength pillars and gratingstructures can be used to form anti-reflecting structures for zero^(th)order diffraction or to form plasmonic structures. The zero^(th) orderdiffraction from such sub-wavelength structures can result in evanescentwaves in the imager pixel. These evanescent waves will result inefficient absorption of the light striking the back in the imager pixel.Higher order diffractions in these structures will result in guidedwaves along the back surface but theses will be reflected by theisolation areas. In this manner efficient light trapping and absorptionof infrared light can be achieved.

In other aspects of the present disclosure, various methods of makingphotosensitive diodes, pixels, and imagers, are contemplated. In oneaspect, as is shown in FIG. 12, a method of making a BSI photosensitiveimager device can include forming at least one junction at a surface ofa semiconductor substrate 1202, forming a dielectric region over the atleast one junction 1204, and forming a textured region over thedielectric region 1206, where the textured region includes surfacefeatures sized and positioned to facilitate tuning to a preselectedwavelength of light 1208. The dielectric region thus isolates the atleast one junction from the textured region, and the semiconductorsubstrate and the textured region are positioned such that incomingelectromagnetic radiation passes through the semiconductor substratebefore contacting the textured region. The method also includes couplingan electrical transfer element to the semiconductor substrate 118 suchthat the electrical transfer element is operable to transfer anelectrical signal from the at least one junction. In one aspect,multiple pixels can be associated together to form an imager. Adielectric region can also be disposed on the backside of thephotosensitive imager device to protect and/or reduce the dark currentof the device. The method can also include coupling an electricaltransfer element to the semiconductor substrate 1210 such that theelectrical transfer element is operable to transfer an electrical signalfrom the at least one junction.

Of course, it is to be understood that the above-described arrangementsare only illustrative of the application of the principles of thepresent disclosure. Numerous modifications and alternative arrangementsmay be devised by those skilled in the art without departing from thespirit and scope of the present disclosure and the appended claims areintended to cover such modifications and arrangements. Thus, while thepresent disclosure has been described above with particularity anddetail in connection with what is presently deemed to be the mostpractical embodiments of the disclosure, it will be apparent to those ofordinary skill in the art that numerous modifications, including, butnot limited to, variations in size, materials, shape, form, function andmanner of operation, assembly and use may be made without departing fromthe principles and concepts set forth herein.

1-16. (canceled)
 17. A method of making a backside-illuminatedphotosensitive imager device, comprising: forming at least one junctionat a surface of a semiconductor substrate; forming a dielectric regionover the at least one junction; forming a textured region over thedielectric region, wherein the textured region includes surface featuressized and positioned to facilitate tuning to a preselected wavelength oflight, wherein the dielectric region isolates the at least one junctionfrom the textured region, and wherein the semiconductor substrate andthe textured region are positioned such that incoming electromagneticradiation passes through the semiconductor substrate before contactingthe textured region; and coupling an electrical transfer element to thesemiconductor substrate such that the electrical transfer element isoperable to transfer an electrical signal from the at least onejunction.
 18. The method of claim 17, wherein forming the texturedregion is by a process selected from the group consisting of plasmaetching, reactive ion etching, porous silicon etching, lasing, chemicaletching, nanoimprinting, material deposition, selective epitaxialgrowth, lithography, and combinations thereof.
 19. The method of claim17, wherein forming the textured region further includes: depositing amask on the dielectric region; etching the dielectric region through themask to form surface features; and removing the mask from the dielectricregion.
 20. The method of claim 19, further comprising etching thesurface features to round exposed edges.
 21. The method of claim 17,further comprising depositing a reflecting region on the texturedregion.
 22. The method of claim 17, further comprising coupling a lensto the semiconductor substrate at a surface opposite the at least onejunction, wherein the lens is positioned to focus incidentelectromagnetic radiation into the semiconductor substrate.
 23. Themethod of claim 17, wherein forming the textured region furtherincludes: depositing a first semiconductor material on the dielectricregion; texturing the first semiconductor material to form a mask;depositing a second semiconductor material on the mask; and etching thesecond semiconductor material to form the textured region.
 24. Themethod of claim 23, wherein texturing the second semiconductor materialfurther includes: etching the second semiconductor material to form aplurality of scallops pointing toward the semiconductor substrate. 25.The method of claim 24, wherein the first and second semiconductormaterials includes a member selected from the group consisting ofsilicon, polysilicon, amorphous silicon, and combinations thereof. 26.The method of claim 17, further comprising depositing an anti-reflectivelayer on the semiconductor substrate at a surface opposite the at leastone junction, such that incident light passes through theanti-reflective layer prior to contacting the semiconductor substrate.27. The method of claim 26, further comprising forming at least oneisolation feature in the semiconductor substrate, the at least oneisolation feature being positioned to reflect light impinging thereonback into the semiconductor substrate.